Hydraulic suspension dampers typically comprise a tube filled with working liquid, inside of which a slidable piston assembly is placed. The piston assembly is attached to a piston rod led outside the damper through the piston rod guide, and comprises a piston with rebound and compression valve assemblies, which control the flow of working liquid passing through the piston assembly during the rebound and the compression stroke of the damper. Some dampers comprise also a base (bottom) valve assembly with separate rebound and compression valve assemblies controlling the flow of working liquid passing in and out of the compensation chamber, usually formed between the inner and the outer tube of the damper.
Each valve assembly usually comprises a stack of resilient disks, often with an additional compression spring, covering the flow passages of the piston and acting as one way valve, deflecting or moving under the pressure of the working liquid to allow the medium flow. Number, shape, diameter, and thickness of each disk provide, among others, an adjustable compression and rebound damping forces.
Typical damper characteristic of damping force vs. piston velocity is a trade-off between improvement of the car handling properties and reduction of the unwanted car vibrations (a so called NVH—Noise, Vibration, Harshness requirements). Although dampers featuring low compression forces with degressive characteristics are required to improve the passengers comfort, during severe road and/or drive conditions they also often lead to maximally admissible wheel-knuckle displacements in damper compression direction leading to a suspension closure or jounce bumper engagement, which in turn affects the car safety, comfort, durability, and noise issues.
From the state of art, there are known dampers in which the damper compression force increases rapidly after reaching a certain velocity of the piston relative to the damper tube.
An exemplary damper of this type, disclosed in patent specification EP 1 215 414 B1, comprises a valve body fixed to the piston rod and urged away from a valve seat surface formed on the piston by a spring. The valve body has a conical surface which moves toward the valve seat surface as a function of dynamic pressure of a damping medium, bearing against the circular valve seat surface and making a linear contact in a closed position. The valve body is made of a plastic material and its outer diameter corresponds to the diameter of the damper tube. To prevent undefined leakage flows the valve body is sealed to the piston rod.
Another damping unit of this type, to be employed in particular for damping the movement of the vehicle steering device, is disclosed in patent specification EP 0 409 094 B1. The unit comprises two annular valve members allocated to each side of valve seat faces of the piston and urged towards the respective opening positions by a plurality of compression springs provided within axial bores of the piston, wherein at least one face of a pair of the piston valve seat face and the valve face allocated to each other is provided with axial projections and recesses, which in a damping position provides a restricted flow communication for working liquid.
Yet another damper having an additional damper valve is disclosed in patent specification EP 1 538 367 B1. This damper comprises a multipart control slide with a pressure-actuated surface, which can move in a closing direction to close a throttle, where the throttle point is determined by the outside diameter of the control slide and an inside wall of the damper tube. The control slide of this invention must be manufactured very precisely in order to achieve its proper operation and to minimize this issue it has a plastically deformable adjusting area.